1. Field of the Invention
The present invention relates to an alkaline storage battery which has a hydrogen-storing negative electrode mainly composed of a hydrogen-storing alloy powder capable of reversibly occluding and releasing hydrogen, and a positive electrode mainly composed of a metal oxide, and a method for producing the battery.
2. Description of Related Art
It is proposed in JP-62-211862A that the utilization of active material in charging and discharging in nickel-cadmium storage batteries can be improved by adding a small amount of yttrium oxide to a powder, mainly composed of cadmium oxide, constituting the negative electrode. However, this patent publication does not disclose the use of yttrium oxide in a hydrogen-storing alloy negative electrode. This patent publication merely shows enhancing the capacity of a battery by improving the utilization of a cadmium negative electrode.
It has further been proposed to add rare metals as an alloy component to a hydrogen-storing alloy negative electrode, but it is not disclosed to inhibit oxidation of the hydrogen-storing alloy by the addition of rare metals. U.S. Pat. No. 5,304,345 to Fujitani et al proposes using yttrium as one of the constituents of hydrogen-storing alloys. This patent does not disclose preparing a negative electrode from a hydrogen-storing alloy powder by adding yttrium or a yttrium compound to the powder.
On the other hand, alkaline storage batteries having a hydrogen-storing electrode as a negative electrode have recently been noted as batteries providing a higher energy density and being pollution-free as compared with conventional secondary batteries such as nickel-cadmium batteries and lead acid batteries.
Such alkaline storage batteries, in which a hydrogen-storing electrode is used as a negative electrode, undergo oxidation of the hydrogen-storing alloy powder which constitutes the negative electrode after repetition of charging and discharging cycles. As a result, the performance of the batteries deteriorates. Usually, the oxygen gas generated from a positive electrode during the overcharging cycle accelerates the oxidation of the hydrogen-storing alloy. Consequently, the charge acceptance of the hydrogen-storing alloy decreases and the hydrogen pressure increases during charging to bring about an increase of internal cell pressure and actuation of a safety valve. Thus, electrolyte is lost and internal resistance of the batteries increases to result in reduction of discharge capacity of the battery after subjected to charging and discharging cycles.
In order to inhibit the oxidation of the hydrogen-storing alloy powder, a method has been proposed to make a negative electrode using a hydrogen-storing alloy powder coated with carbonaceous materials or to coat the surface of the powder with a metal such as copper or nickel by plating or vapor deposition.
Use of the above negative electrode comprising a hydrogen-storing alloy powder which is coated with carbonaceous materials, copper or nickel improves oxidation resistance of the electrode per se and restrains oxidation of the hydrogen-storing alloy powder and, as a result, prolongs the life of the electrode.
However, according to the method of coating the surface of the powder with carbonaceous materials, copper or nickel, cracks occur in the powder of the hydrogen-storing alloy, after repetition of charging and discharging, to produce a fresh active surface. This fresh active surface is oxidized so that the hydrogen-storing ability of the hydrogen-storing alloy decreases. Thus, the method is not satisfactory for restraining the oxidation.